Radiation spectrometers perform pulse light analysis of pulse signals from a radiation detector. The pulse height is measured by detecting the peak values of the pulses. The peak detection involves two signals—peak detect and peak value. In general, the peak value is referred to the maximum of the pulse waveform. It is, however, beneficial to know both the minimal (MIN) and maximal (MAX) peak values of the signal, as is described in V. Jordanov and G. F. Knoll, “Digital Pulse Processor Using A Moving Average Technique”, IEEE Trans. Nucl. Sci., Vol. 40, No. 4, pp 764–769, August 1993; and H. Sawata and Y. Tomimitsu, “Digitalized Amplitude Detection Circuit For Analog Input Signal”, U.S. Pat. No. 4,769,613.
The MAX peak value is used to address a channel in the spectral memory that is incremented. The increment process is initiated by the peak detect signal. In a time variant system, the peak value is the signal value when the peak detect signal becomes active. That is, the pulse waveform is sampled at the activation of the peak detect signal. A similar approach can be used with time invariant systems, but the peak value capture becomes sensitive to the time jitter of the peak detect signal.
The MIN peak value can be used to estimate the noise of the pulse waveform. This is done, for example, by averaging, MIN peak values. The average is used to set the noise threshold of the spectrometer.
Both analog and digital pulse processors use peak detectors. The analog peak detectors typically use a complex scheme of external digital signals to activate and reset the peak detector. Digital peak detectors can be built in a similar fashion but it is advantageous to implement a self-triggered peak detection scheme, as is described in Jordanov, supra, and V. T. Jordanov, “Some Digital Techniques for Real Time Processing of Pulses from Radiation Detectors”, Ph.D. dissertation. The University of Michigan, Ann Arbor, Mich., March 1994.
Hereafter, the pulse signal is assumed to be discrete. The pulse signal samples change at the active edge (e.g LOW-to-HIGH transition) of the system clock (CLK). In addition, for simplicity, the active state of the level signals is HIGH and the inactive state is LOW. Note that these assumptions are only for clarity and simplicity of the description
A common approach to find a MAX pulse peak is to use a low-level discriminator. FIG. 1 depicts a block diagram of a low-level discriminator based peak detector. A first digital comparator CMP1 30 controls the peak detection process. The discrete pulse signal is connected to one of the inputs (A) of comparator 30, while a threshold value is applied to the other input (B). When the discrete signal is below the threshold value, the output of comparator 30 is in inactive state (LOW). When the comparator CMP1 output is inactive, peak register PREG 40 is held in reset state—the output thereof is forced to zero. A second digital comparator CMP2 50 is used to compare the output of peak register 40 with the discrete pulse signal. The output of second comparator 50 is HIGH when the pulse signal sample is greater than the PREG value. The output of second comparator CMP2 50 controls the enable input of peak register 40. When the enable signal is HIGH, the current pulse signal value at the input of peak register 40 can be stored. When the peak register is in a reset state, the enable input is disregarded.
When the discrete pulse signal exceeds the threshold, the output of first comparator CMP1 30 becomes active PREG 40 starts tracking the maximum of the discrete pulse signal. The output of PREG 40 is updated only if the current sample of the discrete pulse signal is greater than the PREG output value. When the pulse signal becomes smaller than the threshold, the first comparator CMP1 30 latches the output of PREG 40 into a latch MAXL 60 and puts peak detector 40 in a reset state. The Peak Detect signal output of first comparator CMP1 30 is the transition of the CMP1 from active to inactive state—HIGH to LOW transition.
FIGS. 2a and 2b illustrate the operation of the low-level discriminator-based peak detector of FIG. 1. Two pulses that partially overlap are shown in FIG. 2a together with the noise threshold. The output of PREG 40 (FIG. 1) is shown in the second waveform in FIG. 2b. The MAX peak value that will be captured is indicated. The last waveform in FIG. 2b is the peak detect signal. It is clear that this type of peak detector detects the absolute maximum while the signal is above the threshold and FIGS. 2a and 2b illustrate a limitation of the low-level discriminator approach; namely, that only one peak over the threshold is detected, even though the resulting pulse signal comprises two pulses each having its own MAX pulse peak.
FIG. 3 shows a modified configuration of the low-level discriminator based peak detector, generally indicated by the reference numeral 70, with MAX and MIN peak values detection. PREG 40 tracks the maximum values when the output of first comparator CMP1 30 is HIGH, while the minimum values are obtained when the CMP1 is inactive. The HIGH to LOW transition of the peak detect signal indicates the capture of the MAX values. The LOW to HIGH transition of the peak detect indicates capture of the MIN detector. An exclusive OR gate receives as inputs the output signals A>B of first and second comparators 30 and 50 and provides and enables the output of PREG 40 to latch into either latch MAXL 60 or MINL 90, depending on whether MAX or MIN peak values have been detected.
Although peak detector 70 has good noise immunity, the throughput rate is reduced due to the fact that partially overlapping pulses cannot be distinguished, as was the case with low-level discriminator 20 (FIG. 1) Note that even partially overlapping the pulse amplitudes can be free of pile-up. In order to peak detect and acquire partially overlapped pulses, a peak detector capable of detecting local peaks is needed
The simplest approach to detect local peaks is to use a differentiated pulse signal and detect the zero crossing, as is described in V Jordanov and G. F. Knoll, supra Depending on the direction of the zero crossing, either MAX or MIN peak values are detected. A peak detector based on this principle is shown in FIG. 4, where it is generally indicated by the reference numeral 100. When the discrete pulse signal is rising, the output of comparator CMP 30 is HIGH. As soon as the discrete pulse signal becomes zero or starts decreasing, comparator CMP 30 changes its state The HIGH to LOW transition captures the MAX peak value while the LOW to HIGH transition captures the MIN peak value.
The operation of differentiation-based peak detector 100 (FIG. 4) is illustrated with the waveforms shown in FIG. 5. The same discrete pulse signal as in the previous case is shown. The second trace shows the differentiated signal. At each crossing of the zero line, the peak-detect signal changes its state. It is obvious that the noise immunity of such detector is very poor. However, it is possible to detect local MAX and MIN values, even ones with very small amplitude.
A modification of peak detector 100 (FIG. 4) is shown in FIG. 6, where it is generally indicated by the reference numeral 110 In this case, the sign bit of a subtractor 120, connected to receive as inputs the output of PREG 40 and the discrete pulse signal, that actually performs the differentiation of the discrete pulse signal indicates the zero crossing. The sign output of subtractor 120 passes through an inverter 130 before serving as latching inputs to latches MAXL 60 and MINL 90. Thus, below we will consider the subtractor sign bit equivalent to a comparator greater (or less) output signal with one input of the comparator connected to zero.
There are modifications of the differential type peak detectors that use either timing or sign bit filtering techniques to improve the noise sensitivity, as described in V. T Jordanov, supra, and F Hilsenrath et al, “A single chip pulse processor for nuclear spectroscopy”, IEEE Trans. Nucl. Sci., Vol. 32, pp 145–149, February 1985.
Although, these methods provide improved performance, the optimal setup is difficult to realize. The timing protection is hard to predict, especially considering the timing walk and timing jitter of the circuits—they also depend on the noise level. In order to optimize the performance of the peak detector, a novel peak-detector configuration has been developed.
Accordingly, it is a principal object of the present invention to optimize the performance of a digital peak detector.
It is a further object of the invention to provide apparatus and method for detecting local maximum and minimum of a detector input signal.
It is another object of the invention to provide apparatus and method for detecting local maximum and minimum of a detector input signal with threshold and hysteresis
Other objects of the present invention, as well as particular features, elements, and advantages thereof, will be elucidated in, or be apparent from, the following description and the accompanying drawing figures.